Opioid receptors display significant differences in membrane trafficking in response to clinically relevant analgesic drugs, and drugs of abuse. Opioid drug-mediated membrane trafficking of opioid receptors is proposed to contribute to the mechanisms underlying addiction and tolerance to opioid drugs. Membrane trafficking of opioid receptors has been studies primarily in non-neuronal cell culture models, and very little is known about the fate of opioid receptors after endocytosis in neurons. Preliminary studies suggest that opioid receptor trafficking in neurons is analogous but not identical to that in fibroblasts in vitro. The studies described in this proposal seek to utilize quantitative immunofluorescence and biochemical based assays to investigate the trafficking of internalized Mu opioid receptor (MOR) in physiologically relevant striatal neurons. The specific aims of the proposal are: 1. Analyze the sorting of MOR between recycling and non-recycling membrane pathways in cultured striatal neurons. 2. Identify sequences in the cytoplasmic domain of MOR that regulate post-endocytic sorting of the receptor in neurons 3. Investigate the functional consequences MOR sorting in neurons, focusing on the relevance of the non-recycling pool of MOR in neurons. These studies will further our understanding of the role of membrane trafficking in regulation of cellular responses to opioid peptides and drugs.